1. Field of the Invention
This invention relates to a fine copper powder suitable for use as filler in a conductive paste or the like, particularly to such a copper powder having improved weatherability.
2. Background Art
Conductive pastes are widely used for forming electronic circuits and the external electrodes of ceramic capacitors. Typical conductive fillers used in conductive pastes include copper, nickel, silver and the like. Among these, copper is used extensively nowadays because it is inexpensive, low in resistance and excellent in anti-migration property. A conductive filler comprising a mixture of copper powders of various particle diameters is usually used in a conductive paste for the external electrodes of a ceramic capacitor. However, in order to form a dense film for improving electrode reliability, the copper powder prior to mixing needs to be one of high fineness, e.g., of a particle diameter of not greater than 0.5 μm, and of uniform particle size.
Methods available for copper powder production include, for example, the atomization process, mechanical crushing process, electrolytic deposition process, vapor deposition process and wet reduction process. The wet reduction process is the main one used today because it is superior in the point of enabling efficient production of a copper powder that is composed of fine spherical particles having a narrow particle size distribution and, as such, is suitable for use in a conductive paste. For example, the prior art includes processes for obtaining fine copper powder by using hydrazine to reduce copper oxide, as taught by JP 10-330801A, JP 1-290706A and JP 5-57324B.